When a superconductor is in contact with a normal material, in equilibrium, the Fermi level of the normal material aligns with the centre of the gap of width 2 .DELTA. between the quasiparticle energy E.sub.Q and the pair energy E.sub.S associated with a superconductor. A barrier region exists between the normal and superconducting regions, the height of which depends upon the two materials involved and the fabrication procedures used. In most junctions which have been fabricated hitherto, the barrier is sufficiently high or wide enough that current cannot flow until the magnitude of the applied bias is greater than half the superconductor gap .DELTA., bringing the Fermi level of the normal conducting material E.sub.1 in line with either the pair energy E.sub.S or the quasiparticle energy E.sub.Q.
For appropriate material conditions, a different process known as Andreev reflection can occur. In this process, when an electron is incident upon the barrier, from within the normal material, it can propagate into the superconducting material only if it pairs with another electron, usually from the normal side. To conserve energy and momentum, a reflected hole, with reverse group velocity and negative effective mass is formed and travels back along the original electron path. Thus, two electrons can simultaneously tunnel across the barrier, one from the Fermi energy and one from an energy level as much below the pair energy as the Fermi level is above. This give an excess current, when compared with normal electron transport since pairs of electrons are formed in the superconducting material. A review of Andreev reflection is given in A. Andreev, Zh, Eksp Teor, Fiz 46, 1823 (1964) [Sov. Phys.-JETP 19, 1228 (1964)].
Hitherto, systems involving two such junctions have not been considered and the present invention resides in the realisation that by the provision of two such junctions, electron transport can occur with the application of a much lower offset bias than hitherto.